The invention is based on a fuel injection pump as defined hereinafter. In a fuel injection pump of this type, known from German Offenlegungsschrift 39 26 166, the fuel injection quantity is divided into a preinjection quantity and a main injection quantity during the high-pressure pumping. This provision makes it possible to decrease the prestorage of uncombusted fuel during the ignition delay and accordingly to avoid overly high pressure peaks in the combustion chamber upon sudden combustion of the prestored fuel, which in turn lessens the thermal and mechanical strain on the engine and lowers the noise it produces.
For these purposes, the known fuel injection pump, which includes a pump piston that for adjusting the injection quantity is guided axially movably and rotatably in a cylinder liner and whose end face defines a pump work chamber that can be made to communicate, via a conduit in the pump piston and two control openings in the cylinder liner, with a low-pressure chamber that surrounds the cylinder liner and forms a suction chamber, is provided with an annular groove in the inner wall of the cylinder liner, above the control openings, on the side toward the pump work chamber. This annular groove cooperates with a recess, which in addition to the usual control recesses is disposed on the jacket face of the pump piston and is defined on all sides by the pump piston jacket face, and also cooperates with a stepped end face of the pump piston that determines the supply onset; beginning at the stepped end face, a longitudinal groove on the pump piston jacket face discharges directly into the annular groove, while the axial length of the recess enables communication during a portion of the piston stroke between the annular groove in the cylinder liner and one of the control openings, with which it cooperates via control edges. Via this communication, during a portion of the supply stroke, the fuel, which is at high pressure, can flow out of the pump work chamber into the low-pressure chamber, resulting in a brief pressure relief in the pump work chamber that in turn causes an interruption in the injection process.
With this arrangement, however, only a constant preinjection onset, a constant preinjection quantity, and a constant pumping interval between the preinjection and the main injection are possible. It is also already known to provide a load- or rpm-dependent shift in injection onset in fuel injection pumps, with the aid of structural provisions at the pump piston, such as chambers on its end, or with reciprocating slides that are axially displaceable on the pump piston. However, to achieve an optimal course of combustion, in terms of noise abatement on the one hand and emission and fuel consumption figures on the other, the injection onset, the preinjection quantity and the injection or pumping interval between the preinjection and main injection must all be optimized as a function of both load and rpm; thus the known fuel injection pump is unable to meet the stringent demands made of a modern internal combustion engine.